The present invention relates to the monitoring of airfield lighting systems and more particularly to a monitoring device for continuously detecting the amount of defective lights in an airfield lighting system.
Airfield lighting systems use lights installed inter alia along runways, taxiways and threshold edges of an airfield to serve as visual navigation aids for the pilots on aircrafts. These lights are commonly coupled to a plurality of isolating transformers which have their primary circuits serially interconnected in a loop circuit supplied by a constant current AC power source, each light being connected to the secondary circuit of a respective isolating transformer. In such plants, the amount of defective lights is never allowed to exceed a few units as required by the security standards for aviation. In order to meet these requirements it is necessary to provide for a continuous monitoring of the airfield lights with a view to detect immediately the amount of lights which become defective.
When a light become defective, the secondary circuit of the isolating transformer to which it is connected is set in open-circuit condition so that a short voltage peak is then produced across the said isolating transformer during the time when the instantaneous induction therein is reversed. The voltage peak is superimposed on the sum of the voltages across all the isolating transformers of which the secondary circuits are still in closed-circuit condition, that is the transformers which are coupled to lights that are still in operating condition.
It can be shown that the integral of such a voltage peak depends very little on the waveform and on the amplitude of the loop current so long as said current is much higher than the saturation current in the isolating transformers, which is practically always the case in airfield plants. When several lights are defective, several voltage peaks are thus superimposed on one another and the integral of these voltage peaks is representative of the number of isolating transformers which have their secondary circuits in open-circuit condition, that is an image of the number of defective lights. It is therefore possible to determine the amount of defective lights by measuring the integral of said superimposed voltage peaks.
A device for generating an output voltage indicative of the number of isolating transformers in open-circuit condition is described in U.S. Pat. No. 4,323,841 issued to Lelf V. Jacobsen. In this prior art device, the output voltage is the voltage integral of the loop voltage from the time when the loop current in each half period passes the zero value and the time when the loop current attains a value corresponding to the saturation current for the isolating transformers. This device has the disadvantage that the edges of the integration window cannot be determined accurately in advance due to the constructional and operational variations from one transformer type to another. In practice, a sophisticated apparatus, e.g. an oscilloscope, is required to adjust each device individually on the spot in order to settle the beginning and end edges of the integration window when the current passes the zero value and attains the saturation value respectively. Moreover, these edges cannot be readily recognized, especially when the loop is supplied by a resonant type current regulator.
This inaccuracy in determining the width of the integration window is detrimental to the detection accuracy. Indeed, in order to obtain a high detection accuracy it is most desirable to determine the voltage integral over a time window during which the superimposed voltage peaks have a significant value. Further, the said inaccuracy in determining the width of the integration window is all the more detrimental to the detection accuracy as there is a requirement for detecting a low percentage of defective lamps and especially when the loop is supplied by a resonant current regulator which provides a quasi-sinusoidal output current.